An optical measuring device relies on a technique consisting in measuring the frequency shift, representative of the speed relative to the air, between a laser beam emitted in the atmosphere and the beam backscattered by the natural aerosols in the air, these aerosols being used as wind field tracers. This technique is referred to as longitudinal laser Doppler anemometry since the frequency shift that results from the Doppler effect is directly proportional to the projection of the velocity vector on the line of sight.
The useful information carried by the Doppler frequency shift, hereinafter called the Doppler shift, is obtained by carrying out coherent-type detection: a beam coming from a coherent light source, for example a laser, is split into two beams. A first beam, called signal beam, is sent into the measurement zone and a second beam, called reference beam or local oscillator, constitutes a reference for detecting the Doppler shift.
The aerosols naturally present in the atmosphere backscatter the light of the signal beam, producing a signal backscattered by the medium, the frequency of which undergoes a Doppler shift ΔfDoppler relative to that of the incident light. The signal backscattered by the medium interferes with the reference beam on the photosensitive surface of a detector. The frequency of the electrical signal delivered by the detector corresponds to the difference ΔfDoppler between the frequency of the backscattered signal and the frequency of the reference beam, and from this a measurement of the relative speed of the aircraft, i.e. relative to the medium, is deduced knowing that the expression linking these two quantities is the following:ΔfDoppler=2v/λ  (A)                v being the projection, on the line of sight of the laser, of the velocity vector of the aircraft relative to the ambient medium (the atmosphere); and        λ being the wavelength of the emitted beam in the medium.        
The components of the velocity vector {right arrow over (V)} of the aircraft relative to the ambient medium are determined by measuring, possibly sequentially, projections of the velocity vector of the aircraft relative to the ambient medium in at least three noncoplanar directions.
FIG. 1 shows a block diagram of a device for optically measuring the Doppler shift ΔfDoppler, constituting the prior art of a heterodyne laser anemometer.
The device of FIG. 1 comprises a laser unit ULAS_A 10 delivering a light beam as input to a splitter unit USEP_A 20 delivering a signal light beam Fs as input to an optical signal transceiver system EMIREC 50 and a reference light beam Fr as input to an optical coupler MEL 30.
The laser unit ULAS_A comprises a radiation source and an optical device for spatially shaping the radiation emitted by the source. The laser unit ULAS_A produces a light beam, the wavelength λ of which is for example 1.55 μm, this being a wavelength commonly employed in the optical telecommunications field as the atmosphere is relatively transparent at said wavelength.
The various constituents of the laser unit ULAS_A are not shown in FIG. 1.
The optical signal transceiver system EMIREC comprises in series an optical signal amplifier BOOS 53, a splitter unit USEP_B 54 and a displacement unit UDP 55 delivering an optical power signal Sinc focused in a focusing zone ZOF within the reference medium MILREF 60. The optical signal transceiver system EMIREC may also include an optical signal frequency shifter DEF 51, for example an acoustooptic modulator, which shifts the frequency of the beam that is applied to it by around one hundred megahertz.
The displacement unit UDP is characterized by an optical focus Fopt and an optical axis denoted by X, these being shown more explicitly in FIG. 2. The term “line of sight” LDV is given to the axis joining the optical focus Fopt to the center of the focusing zone ZOF where the optical power signal Sinc is focused. The YZ plane is normal to the X axis.
The orientation of the line of sight LDV, which is also the preferential orientation of the optical power signal Sinc emerging from the displacement unit UDP, may be controlled. This is also the case for the distance separating the focus Fopt from the center of the focusing zone ZOF, as shown in FIG. 2.
The splitter unit USEP_B comprises for example, in series, a polarization splitting coupler followed by a two-way optical link. The various constituents of the splitter unit USEP_B are not shown in FIG. 1.
The displacement unit UDP captures light rays Sr backscattered by the reference medium MILREF in a specified direction.
The backscattered light rays Sr may possibly have a Doppler shift ΔfDoppler generated by the medium MILREF relative to the incident beam Sinc. The backscattered light rays Sr are captured by the displacement unit UDP—they take the form of a backscattered signal beam Sr, also called a “light echo”, which is transported through the splitter unit USEP_B before entering the optical mixing coupler MEL.
The optical mixing coupler MEL receives, on a first input, the reference light beam Fr coming from the coupler USEP_A and, on a second input, the backscattered signal beam Sr coming from the splitter unit USEP_B. The optical mixing coupler MEL mixes the two optical signals applied to its two inputs, which produces periodic beating on the photosensitive surface of a detector DET 40.
The detector DET delivers an electrical signal when a light beam of wavelength λ is applied on its sensitive surface. The electrical beat signal produced by the detector DET, when its sensitive surface is illuminated by the periodic beating, varies at the same frequency as the periodic beating.
A signal processing unit UTR 70 receives the electrical beat signal resulting from the beating between the reference beam Fr and the backscattered beam Sr and enables its Doppler frequency ΔfDoppler to be estimated.
The measurement of the projection on the line of sight LDV of the aircraft's relative velocity vector with respect to the medium {right arrow over (V)} is derived from the measurement of the Doppler shift ΔfDoppler.
FIG. 2 details the operation of a displacement unit UDP 57 comprising an optical focusing system SOF 56 and a deflection unit UD.
The optical focusing system SOF 56 controls the value of the distance between the focusing zone ZOF of the optical power signal Sinc and the focus Fopt the displacement unit UDP along the direction of the line of sight LDV. The optical focusing system SOF may for example be an optic of variable focal length, the value of the focal length being, in this case, determined by means of an electrical focusing control CEF.
The deflection unit UD controls the orientation of the line of sight LDV of the optical power signal Sinc. The deflection unit UD may for example be a prism rotating about the optical axis of the optical focusing system SOF. The movement of the prism is for example controlled by means of an electrical scan control signal CEB. The control signal CEB sent to the deflection unit UD acts on the orientation of the line of sight LDV and consequently on the position of the focusing zone ZOF in the reference medium MILREF.
In the prior art, a device for measuring the velocity vector {right arrow over (V)}, similar to that shown in FIG. 1, performs an elementary measurement of a projection of {right arrow over (V)} along at least three selected, noncoplanar, elementary orientations of the line of sight LDV. In this case, the elementary measurements are performed sequentially and the line of sight LDV is kept unchanged in each of the three selected elementary orientations of the line of sight LDV during the elementary measurement. The determination of the velocity vector {right arrow over (V)} is deduced directly from the elementary measurements by a geometric calculation that depends only on the elementary orientations of the line of sight LDV. The movement of the line of sight LDV between the measurements serves to rally, rapidly and in succession, the elementary orientations of the line of sight LDV.
One of the main figures of merit of the anemometer shown in FIG. 1 is the signal-to-noise ratio SNR measured on the output of the detector DET. The higher the SNR, the easier it is to perform the anemometric measurement. The SNR is higher the greater the amount of energy of the light echo Sr arriving on the detector of the detection unit UDET.
For an optical power signal Sinc of given energy incident in the medium MILREF, the light echo Sr is stronger the greater the concentration of efficiently backscattering aerosols of the medium MILREF in the focusing zone ZOF. Now, the aerosol concentration greatly decreases with altitude. Consequently, to be capable of operating just as well at high altitude, i.e. above 4 kilometers, as at low altitude, a laser anemometer must conventionally emit, in the medium MILREF, a strong optical intensity Sinc along a fixed direction, which poses safety problems, especially eye safety problems, in particular under flight conditions at low altitude.